Unlike woodwind instruments with reeds, flutes are aerophones or reedless wind instrument that produces sound from the flow of air across an opening. Nearly all musical instruments comprise two basic elements: a generator that produces a vibration, and a resonator that amplifies the vibration and modifies it to create the sound of the instrument. The generator on a flute is the mouth hole edge against which the player's breath is directed. The stream of air from the player's lips travels across the embouchure-hole opening and strikes against the sharp further edge of the hole. When the airstream meets the edge it does not divide into two separate airstreams. Instead, a wave-like displacement travels along it and deflects it causing the airstream to rapidly fluctuate between going into the hole and going away from the hole, as shown in FIG. 1. This sets up a rapid vibration at the head of the tube.
The speed of this displacement wave on the air stream is about half the airspeed of the airstream itself (which is typically in the range 20 to 60 meters per second, depending on the air pressure in the player's mouth). If the airstream speed is carefully matched to the frequency of the note being played, then the stream will flow into and out of the embouchure hole at its further edge in just the right phase to reinforce the sound and cause the flute to produce a sustained note.
The Western concert flute is a transverse treble flute that is closed at the top. An embouchure hole is positioned near the top. The hole is covered and surrounded by a lip plate, which is a curved, oval-shaped metal plate with a hole aligned over the embouchure hole. The flute has circular tone holes larger than the finger holes of its baroque predecessors. The size and placement of tone holes, key mechanism, and fingering system used to produce the notes in the flute's range were evolved from 1832 to 1847 by Theobald Boehm.
Precise control of the airstream over the embouchure hole is crucial for producing notes with a flute. Unfortunately, because of the open airstream passing over the embouchure hole the flautist is at the mercy of ambient air currents. In windy conditions, for example in outdoor concerts or marching band shows, control of the airstream is difficult if not impossible, effectively silencing the flute. Even in indoor conditions flute players sometimes encounter trouble playing under or next to fans, HVAC vents, or close to windows or other areas where there are heavy air currents. In the past, flutists have tried to mitigate the problem with makeshift measures such as surrounding themselves with music stands, turning backs to audiences to avoid wind gusts, etc., none of which is optimal.